Monthly Archives: August 2015

2014-2015 Review

Mr Reid travelling salesman problem (2000 nodes)

I had a very busy timetable at work this year, so I didn’t blog here as much as I would have liked. Hopefully the 2015-2016 Review will be longer than this one.

I enjoyed writing about rotational kinetic energy in September, and the difference between centre of mass and centre of gravity was something people don’t often think about.

A post on how latitude affects the length of the day was my favourite post from October.

November was one of my favourite months: I particularly liked the posts about fail-safes, citrus fruit and analemmas.

I only made two posts in December, about the colour of diamonds and the possibility of building a vacuum airship, but I liked them both.

January was also a good month. My favourites were two posts about ranking things properly, and the different types of microscope. I also wrote about different naming systems across the world, but I’m not sure I got my facts 100% correct.

A post about the thermal expansion of petrol was easily my favourite post from February.

I liked all the posts I made in March, particularly the posts about colour mixing (why is light red, green and blue, but ink cyan, magenta and yellow?), the order of adjectives in English, and alternatives to GPS.

April was also a good month: I wrote about the nuclear “double flash” and worked out a much better system of coinage for the UK.

A post about how to look at the back of your head using a black hole was my favourite post from May.

I didn’t make any posts in June. :(

I particularly like my post about why some particles don’t decay from July. I also wrote about the different type of multiplication and the world’s longest golf shot.

August was another good month (school holidays!). I wrote about the US’s “soft bomb”longitude, latitude and precision and the Kp index for aurora spotting.

Here’s to 2015-2016. I hope you’ll keep reading.

Soft Bomb

A soft bomb (also known as a “blackout bomb”) is a bomb designed to destroy electrical power infrastructure rather than physical buildings. The only known soft bomb is the US’s CBU-94 cluster bomb dispenser carrying BLU-114/B warheads (the finer details of which are still classified).

Once dropped by an aircraft, as it is falling, the bomb orients itself vertically and then discards its outer casing and begins spinning. Once it is spinning it releases two hundred drink can-sized submunitions, each of which contains thousands of tiny graphite fibres. A small explosive charge inside each submunition then detonates, scattering the graphite fibres like electrically conducting confetti and forming a “net” that drifts slowly downwards. When this conductive confetti lands on the target – an electrical transformer or substation – it causes thousands of explosive short circuits, starting fires and disabling the target.


Soft bombs was first used in May 1999, as part of US operations in Serbia, in which night-time strikes by F-117A stealth fighters knocked out electricity to 70% of the country. They were then used again a few days later to undo repairs that the Serbians had completed. The photograph above comes from a video shot at the TPP Nikola Tesla power plant that was heavily damaged in the attack.

Longitude, Latitude and Precision

There are two main ways of giving longitude and latitude: in degrees, minutes and seconds, and in decimal degrees. For example: the current headquarters of the Institute of Physics could be given as 51° 31′ 18.0721″N, 0° 8′ 42.8759″W in degrees, minutes and seconds format; or as 51.521687, -0.145243 in decimal degrees. (Note the absence of cardinal directions in the decimal degrees format: positive latitude values are taken to be north, with negative values being south, and positive longitude values taken to be east, and negative values west.)

Lines of longitude are great circles, each one the same size, running around the Earth and crossing both the north and south poles. When quoting the position of an object on Earth’s surface in terms of its longitude, the degree of precision is not affected by the longitude in question.

Precision Error
1.0° / 1.0° 0′ 0″ ± 111 km
0.1° ± 11.1 km
0° 1′ 0″ ± 1.85 km
0.01° ± 1.11 km
0.001° ± 111 m
0° 0′ 1″ ± 30.9 m
0.0001° ± 11.1 m
0.00001° ± 1.11 m
0.000001° ± 0.111 m

Unlike lines of longitude, not all lines of latitude are the same length. At 80° latitude the length of the line of latitude is only 17.3% of that at the equator. This means that the level of precision changes with latitude – each division of the latitude line is much smaller at you approach the north pole.


As you can see in the graph above, the change is not linear with latitude. The same 35° difference in latitude yields very different results in terms of accuracy.

This means that different locations will need to use different degrees of accuracy in their GPS coordinates if you want people to be able to find their way around. At Quito, near the equator at 0.23°N, a precision of 0.001° would lead to an accuracy of ± 111 metres, but at Helsinki, much further north at 60.2°N, this accuracy is improved to ± 55 metres.

The Kp Index

The Kp index is a way of quantifying the level of geomagnetic activity, and the chance of observing the aurora borealis. The higher the Kp index the higher the chance of observing the aurora, and the further south the aurora may be visible.



At K= 5 the aurora can be seen from the very north of mainland Scotland, and at K= 7 it can be seen in London (assuming, in both cases, ideal observing conditions).

The Kindex is calculated every three hours by taking the average of the K-index as measured at thirteen different measuring stations. The K-index is a logarithmic scale that measures disturbances of the Earth’s magnetic field caused by solar activity, but it is adjusted so that the regularity of occurrences of each K-index value is the same at each station: that is, the frequency of K= 3 events is the same at Lerwick in Scotland as it is at Witteveen in the Netherlands, even though Lerwick is far more northern than Witteveen.


The disturbances in the Earth’s magnetic field that the K-index measures are important because it is these disturbances that push the particles into the atmosphere, where they ionise the particles there, causing the emission of light that make up aurorae.